Boundless Bio files S-4 for Serapha merger; partner later reports early SERP-01 data

The merger

Boundless Bio, Inc. filed the registration statement on Form S-4 on October 8, 2026, for its planned merger with the privately held Serapha Bio, Inc. The two companies signed an Agreement and Plan of Merger and Reorganization on June 22, 2026 and amended it on August 28, 2026. Boulder Merger Sub Corp. will merge into Serapha, which will survive as a wholly owned subsidiary of Boundless Bio. After closing, Boundless Bio will be renamed Serapha Bio, Inc., and the combined company expects to trade on Nasdaq under the symbol "AATD".

The S-4 estimates that each Serapha share will receive about 8.6737 Boundless shares. That estimate does not reflect the Nasdaq reverse split Boundless plans to seek, and it adjusts with Boundless's net cash at closing. Boundless stockholders are asked to approve the share issuance, which will exceed 20% of the pre-merger shares and result in a change of control under Nasdaq rules, and a charter amendment for the reverse split.

Copies go to Latham & Watkins LLP and Gibson, Dunn & Crutcher LLP.

Ownership, cash and the financing

After the merger, existing Boundless holders are expected to own about 3.8% of the combined company on a fully diluted basis. Former Serapha holders, excluding shares bought in the Serapha pre-closing financing, are expected to own about 29.3%. Shares issued in that pre-closing financing are expected to represent about 66.9%.

Serapha raised about $138 million gross in a Series A preferred private placement tied to the merger signing. It has also agreed to a pre-closing financing for about $92 million gross, under a securities purchase agreement for common stock or pre-funded warrants, and the closing of the merger is conditioned on it. Boundless plans a pre-closing dividend of about $44 million to $48 million, based on the amount by which its net cash at closing is expected to exceed zero.

Boundless's 10-K for 2025 reported a net loss of $58.2 million and cash used in operations of $46.7 million. At June 30, 2026, it held cash and cash equivalents of $20.2 million and short-term investments of $52.4 million, according to its 10-Q.

Why Boundless is doing the deal

The S-4 gives the Boundless board's reasons. It cites the significant decline in the trading price of Boundless stock over the preceding two years and the risks and costs of keeping its Nasdaq listing. The board also believed it would have difficulty raising enough capital alone, citing emerging clinical data for BBI-940 from the KOMODO-1 study. Outreach by Lucid Capital Markets, LLC to approximately 14 potential counterparties produced two non-binding proposals.

SERP-01 and the litigation

The combined company will focus on SERP-01, Serapha's only product candidate, licensed from YolTech. The FDA granted SERP-01 orphan drug designation for PiZZ AATD and regenerative medicine advanced therapy designation in May 2026.

On September 25, 2026, Beam Therapeutics Inc. filed a complaint in the U.S. District Court for the District of Massachusetts against Serapha, YolTech and two YolTech-affiliated individuals, Dr. Zi Jun (Emma) Wang and Dr. Yuxuan (Jensen) Wu. The complaint alleges, among other things, trade secret misappropriation and related claims in connection with the development of SERP-01, and seeks monetary damages and injunctive and other relief. Serapha says the claims are without merit and that it will defend itself vigorously. The S-4 says an adverse outcome could materially affect the combined company's ability to develop and commercialize SERP-01, and that the case is not expected to be resolved before the closing.

The later data release

After the S-4, on October 26, 2026, Serapha published early SERP-01 data in a communication filed by Boundless under Rule 425. It covers an investigator-initiated trial in four participants with the PiZZ genotype. Serapha reports that a single dose raised total serum AAT above the protective threshold in all four and that no serious adverse events or treatment-related events of grade 3 or higher were seen as of the cutoff. The release says longer follow-up and more participants are needed, and chief executive Kenneth T. Mills's "best-in-class" language rests on four patients. Serapha intends to start a U.S. Phase 2/3 trial in late 2026 or early 2027.

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